Modern fuel injectors often have a nozzle portion in which a check or needle valve is used to alternatively start or stop fuel injection. Often the check valve comprises a check valve member biased against the inner wall of the nozzle in such a way as to fluidly isolate the fuel injector orifices from the fuel chamber.
The bias keeping the check valve closed can be provided by a spring, hydraulic pressure pushing against the check valve member, other biasing means, or a combination of these. Fuel injection usually commences when fuel pressure in the fuel chamber surrounding the check valve member becomes great enough to overcome the bias keeping it closed. This can be accomplished by increasing the fuel pressure in the fuel chamber, decreasing the closing bias, or a combination of both.
Accordingly, the check valve member is subjected to high forces during operation. The check valve member is generally sliding against one or more guide surfaces in the nozzle and also impacts and presses against the inner wall of the nozzle, for example a seat in the nozzle. Wear of the check valve member and the nozzle, and accordingly their expected lifetimes, are to a large degree dependent on the friction coefficients of their materials. There is friction on the check valve member as it slides against the nozzle guide surfaces and the seat, or at other areas where the valve member impacts against the nozzle wall. The greatest wear on the check valve member usually occurs where the check valve member impacts the inner wall of the nozzle to close off the orifices.
There have been many attempts to incorporate ceramic materials into fuel injector nozzles and nozzle valve assemblies for various reasons, not necessarily to reduce friction. Sometimes the lighter weight of ceramic materials is found desirable, and sometimes the corrosion resistant properties of ceramic materials are desired.
However, there have been problems associated with trying to use ceramic check valve members in fuel injectors. For example, because of differences in coefficients of expansion with temperature between ceramic materials and the metallic components of the fuel injector (for example 52100 steel) during fuel injector operation, the operating characteristics of a fuel injector using ceramic parts can change dramatically with temperature. For example, a significant change in the operating temperature can lead to unacceptably high changes in guide clearance and in check lift (the distance between the check valve member and the nozzle seat when the check valve member is in opened position).
Many approaches to using ceramic materials in nozzle assemblies have not addressed the possible problems caused by differing coefficients of expansion. For example, U.S. Pat. No. 5,409,165 to Caroll, III et al., issued Apr. 25, 1995, concentrates on the wear resistant properties of ceramics, and teaches a wear resistant plunger assembly for a fuel injector and discusses the possibility of using ceramic material for the plunger tip. As another example, the Bosch Corp. of Germany is believed to use a known diamond-like coating (DLC) similar to ceramics on an upper guide portion of a check valve member in a fuel injector.
But there have been attempts to work around the thermal expansion problem. For example, U.S. Pat. No. 5,607,106 to Bentz et al., issued Mar. 4, 1997, teaches a fuel injector needle valve assembly including a silicon nitride needle tip operating in conjunction with a valve seat subassembly in the nozzle made from a combination of metal and ceramic. Adding ceramic material to the valve seat subassembly may lower the coefficient of thermal expansion of the valve seat subassembly to one similar to that of the ceramic needle tip, thus reducing gross heat expansion differences between the nozzle and the needle tip portions. However, adding ceramic materials to the nozzle to match thermal expansion characteristics of a ceramic check valve member complicates the manufacturing process considerably and introduces additional weaknesses to structural integrity of the nozzle assembly. This makes the fuel injector more costly to manufacture and more likely to fail over time.